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AS1545

Data Sheet - D e t a i l e d D e s c r i p t i o n

Figure 34. Typical Application

Analog Input Composition

The equivalent circuit of analog input structure of the AS1545 in differential/pseudo differential modes is shown in the

Figure 35. In single-ended mode, AIN- is internally tied to AGND. The four diodes provide ESD protection for the

analog inputs. These diodes can conduct up to 10mA without causing irreversible damage to the part.

Note: Make sure that the analog input signals never exceed the supply rails by more than 300mV. This causes the

diodes to become forward-biased and starts conducting into the substrate.

The C1 capacitors in Figure 35 are of 4pF and can be attributed to pin capacitance. The value of these resistors is

typically about 100

Ω. The C2 capacitors are the ADC’s sampling capacitors with a capacitance of 20pF typically.

For ac applications, removing high frequency components from the analog input signal is recommended by the use of

an RC low-pass filter on the relevant analog input pins with optimum values of 10

Ω and 10nF. In applications where

harmonic distortion and signal-to-noise ratio are critical, the analog input should be driven from a low impedance

source. Large source impedances significantly affect the ac performance of the ADC and may necessitate the use of

an input buffer amplifier. The choice of the op amp is a function of the particular application.

Figure 35. Equivalent Analog Input Circuit

When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The

maximum source impedance depends on the amount of THD that can be tolerated. The THD increases as the source

impedance increases and performance degrades.

Figure 23 on page 13 shows a graph of the THD vs. the analog input signal frequency for various supplies in single-

ended mode, while Figure 25 shows the THD vs. the analog input signal frequency for various supplies in differential

mode.

Figure 22 on page 13 shows a graph of the THD vs. the analog input frequency for different source impedances.

AIN 0

AIN 1

REFA

AVDD

DVDD

SCLK

CSN

RANGE

DOUTA

SGL/DIFF

DOUTB

REFB

REFSEL

100nF

1µF

10µF

100nF

1µF

10µF

µC

+

-

4.7µF

4.7µF

10nF

10nF

10

Ω

10

Ω

AS1545

BIN 0

BIN 1

+

-

10nF

10nF

10

Ω

10

Ω

A0 A1 A2

VDRIVE

1µF

100nF

VDD

D

D

C1

AIN+

C2

R1

VDD

D

D

C1

C2

R1

AIN-

Open for conversion,

closed for tracking.

Open for conversion,

closed for tracking.

4pF

4pF